1. Technical Field
The invention concerns a method of digital telecommunication via an electrical cable, and more particularly, a method of digital telecommunications where the symbols of a data stream to be transmitted are supplied as a transmission signal to a transmission path after undergoing a digital-to-analog conversion, where the transmitted signal is sampled at the end of the transmission path and is then further processed and supplied to a decision element, and where a Viterbi decoder operating on the basis of a trellis diagram is used to further process the sampled signal, which uses a number of feedback filters to determine branch and path metrics in the trellis diagram, where this number is a function of the number of states in the trellis diagram, with in-line filter taps according to the Delayed Decision Feedback Sequence Estimation-Algorithm.
2. Description of the Prior Art
Digital telecommunication via band-limited channels, such as e.g. copper cables in a local subscriber line network, are subject to disturbances in the form of intersymbol interferences, noise and pulse noise, which produce a limitation of the maximum transmission range. The widest possible equalization (correction) of the intersymbol interference can be achieved with the help of adaptive equalization methods based on digital filter algorithms. However, the performance capacity of such mostly linear filter algorithms is restricted by limited word lengths of digital signal processing and a limited total complexity, as well as by noise and nonlinearities in the analog circuit portion of the transmitting and receiving devices.
An increase in the transmission range or in the resistance to interference is possible with the help of optimum receiving methods according to the principle of Maximum Likelihood Sequence Estimation (MLSE) as disclosed in the document "Maximum-Likelihood Sequence Estimation of Digital Sequences in the Presence of Intersymbol Interference", G. D. Forney, Jr., IEEE Transactions on Information Theory, volume IT-18, no. 3, May 1972, pages 363 to 378, which provides an example of an MLSE receiver. In this example the transmission channel is considered a "finite state machine". Using a Viterbi algorithm, as disclosed for example in the document "The Viterbi Algorithm", G. D. Forney, Jr., Proceedings of the IEEE, volume 61, March 1973, pages 268 to 277, an estimate of the transmitted signal sequence is obtained by using the channel states according to the Maximum Likelihood criterion.
A problem in principle during the implementation of MLSE algorithms is the proportionality of complexity (number of gates being used) and the number of states in the trellis diagram, which in turn increases exponentially with the channel memory, i.e. with the length of the pulse response in the dispersive transmission channel. The Delayed Decision Feedback Sequence Estimation (DDFSE) algorithm, also known from the document "Delayed Decision-Feedback Sequence Estimation", A. Duel-Hallen, et al., IEEE Transactions on Communications, volume 37, no. 5, May 1989, pages 428 to 436, is a method of decreasing the number of states in the trellis diagram. As with the Viterbi algorithm, in this case as well the states describe all the state variations of the channel considered a "finite state machine" within an established time interval of .mu. symbols. However, while .mu. is identical to the length of the channel memory in the MLSE algorithm, .mu. can be freely chosen in the DDFSE algorithm. This means that a finite .mu. can even be used with an infinitely long channel memory.
Each state of the trellis diagram in the DDFSE algorithm only represents a part of the information about the channel state. The rest of the information is estimated by means of the transmission symbols of the highest probability, which correspond to the preceding path decisions and are stored in a path memory. The DDFSE algorithm combines the properties of the Viterbi algorithm with those of a decision feedback equalization. The DDFSE algorithm can also be imagined with X.sup..mu. feedback filters, where the filter taps of the feedback filters being used are fed different receiving symbols during each symbol interval. In this case X is the number of possible symbols or amplitude stages per step. The combination of symbols in one of the feedback filters, which leads to the smallest error in the decision element, is optimum in the sense of maximum likelihood.
The complexity of the DDFSE algorithm rises exponentially with the parameter .mu., i.e. with the number of symbols considered in its states, or proportionally with the number of states in the trellis diagram. The gain in the DDFSE algorithm increases with increasing .mu. as well as with the increasing length n-.mu. of the feedback filters during the determination of the branch metrics. In that case n indicates the number of filter tap operations for determining the branch metrics per state of the trellis diagram, which in the following is set to be equal to the number of postcursors in the time-discrete channel pulse response. Each filter tap operation comprises an addition, a multiplication and a register operation. The implementation of sufficiently long feedback filters for all states of the trellis diagram requires a large number of logic gates as well as a considerable amount of circuitry.